Stereolithography (SLA)

SLA

Stereolithography (SLA) is the pioneer technology in the field of 3D Printing. The entire 3D Printing journey started with the advent of SLA. There are different variants of the Stereolithography technology with respect to how the parts are fabricated — top-down, bottom-up, CLIP etc. But the setup is almost same irrespective of the fabrication approach. A typical SLA setup consists of a vat of photopolymer resin, a UV laser, and a perforated platform. The entire setup is isolated from ambient air and is enclosed in a chamber. The 3D CAD file is input to the 3D Printer where the parts are sliced into subsequent layers, with each layer height corresponding to the layer thickness setting chosen. The UV laser scans the first layer and cures the layer’s cross-section based on the input data from slicing software. Then the platform descends by one layer thickness value. The average layer thickness value in SLA is 100µm. The sweeper then sweeps a new layer of resin in position. The UV laser cures the new layer, the platform descends again, and the process continues till the part is fabricated completely. After part fabrication the parts are immersed in a chemical bath to get rid of any extra resin residue and the parts are then cured by heating them in an oven.

One should not confuse SLA with Digital Light Processing (DLP) systems. In DLP, instead of a UV laser, a UV projector is used!

Unlike SLS, SLA parts cannot be stacked throughout the volume of the bed, as this will result in support structure generation in the space between the parts stacked one on top of the other. And unlike FDM, more than one part can be fabricated in SLA, provided all the parts fit in one single horizontal plane.

There is support structure generation in SLA technology. The support structures are fabricated in the same material as that of the parent material. But the support structures are optimized by designing and orienting them in such a way that they are less dense and have minimal contact with the actual part’s surface area, thereby facilitating easy removal. The support structure removal usually leaves tiny burr marks on the surface. SLA parts have an inherent smooth surface finish but the parts are usually post-processed (polished) to remove tiny defects (burr marks) if any, left during support removal. This gives the SLA parts an even superior surface finish.

SLA part with support structures

Finer the layer thickness, better the surface quality, but more time is required to fabricate the parts and thus higher the cost, and vice-versa.

Smooth surface finish

SLA parts come with varying layer thicknesses right from 150µm to 25µm. Finer the layer thickness, better the surface quality, but more time is required to fabricate the parts and thus higher the cost, and vice-versa. The average layer thickness in SLA is 100µm. SLA parts can be made hollow to reduce weight, and save material and cost, but an escape hole needs to be provided for the excess resin to drain off.

The most commonly used materials in SLA are PP-equivalent plastic (white, opaque parts) and PC-equivalent plastic (translucent parts). These resins have high viscosity. Since SLA parts are dimensionally very accurate — with an accuracy of around (+/-) 150µm — and have a good surface finish, they are used as master patterns for vacuum casting (silicone molding). SLA parts also find applications ranging from proof of concept models to automotive styling and designing parts.